I'm willing to work OT for free sometimes so long as my employer is ok with me taking a few hours off when there's nothing to do.
Really my employer has one of the best ways to pay people.
You get a bonus based on the hours you work. That way if you work OT you might not get paid for it, but your bonus will thank you in a quantitative way. None of this "hope your boss noticed" crap.
How much is the bonus? If it doesn't come out to what you'd be getting with hourly pay w/ overtime, then you're just kidding yourself.